MOSFET structure with T-shaped epitaxial silicon channel

ABSTRACT

A MOSFET disposed between shallow trench isolation (STI) structures includes an epitaxial silicon layer formed over a substrate surface and extending over inwardly extending ledges of the STI structures. The gate width of the MOSFET is therefore the width of the epitaxial silicon layer and greater than the width of the original substrate surface between the STI structures. The epitaxial silicon layer is formed over the previously doped channel and is undoped upon deposition. A thermal activation operation may be used to drive dopant impurities into the transistor channel region occupied by the epitaxial silicon layer but the dopant concentration at the channel location where the epitaxial silicon layer intersects with the gate dielectric, is minimized.

TECHNICAL FIELD

The disclosure relates to semiconductor devices and manufacturingmethods for the same and, more particularly, to a MOSFET structure witha T-shaped undoped epitaxial silicon channel.

BACKGROUND

MOSFET, Metal Oxide Semiconductor Field Effect Transistor, devices arehighly utilized components in integrated circuit and other semiconductordevices. MOSFETs are used for amplifying or switching electronic signalsand provide functionality to the devices. MOSFETs that include n-typetransistor channels are referred to as n-MOSFETs and MOSFETs thatinclude p-type transistor channels are referred to as p-MOSFETs. MOSFETsmay be formed using various techniques and materials but requireaccurate and precise placement of their various components andconstituents. One of the constituents is dopant impurities that areintroduced into various components of the MOSFET such as the gatestructure, the source and drain regions and the transistor channel. Thecharacteristics of the dopant impurities in each of the aforementionedstructures such as the location and concentration, must be carefullycontrolled.

Heavily doped transistor channels have been favored in the rapidlyadvancing semiconductor manufacturing industry because they enabletransistors to operate at higher speeds. Conventional MOSFET devices,however, suffer from random dopant fluctuations due to the heavily dopedtransistor channels. It would therefore be desirable to correct thisshortcoming of the conventional technology.

BRIEF DESCRIPTION OF THE DRAWING

The present disclosure is best understood from the following detaileddescription when read in conjunction with the accompanying drawing. Itis emphasized that, according to common practice, the various featuresof the drawing are not necessarily to scale. On the contrary, thedimensions of the various features are arbitrarily expanded or reducedfor clarity. Like numerals denote like features throughout thespecification and drawing.

FIG. 1 is a top, plan view of an exemplary MOSFET according to thedisclosure. FIG. 1A is a cross-sectional view taken along line 1A-1A ofFIG. 1 and FIG. 1B is a cross-sectional view taken along line 1B-1B ofFIG. 1;

FIGS. 2A-2D are cross-sectional views showing a sequence of processingoperations used to form an exemplary terraced STI structure according tothe disclosure;

FIGS. 3A-3D are cross-sectional views showing an exemplary method forforming a MOSFET according to the disclosure; and

FIG. 4 is a graphical representation of a dopant impurity profile in thechannel region of an exemplary MOSFET.

DETAILED DESCRIPTION

The disclosure provides a MOSFET having a channel region formed of anepitaxial silicon layer disposed on a substrate and according to oneadvantageous embodiment, the dopant profile of the transistor channelincludes the epitaxial silicon layer being an undoped layer with thechannel dopant impurities species residing in the semiconductorsubstrate beneath the epitaxial silicon layer. The transistor furtherincludes an increased effective gate width provided by a T-shaped crosssection in which a part of the epitaxial silicon layer forming thechannel, overlies a submerged ledge of an STI, shallow trench isolation,structure such that the transistor channel width is larger than theminimum distance between bookend STI structures.

FIG. 1 is a top, plan view showing transistor area 1. Transistor area 1is defined by boundaries 3. Transistor area 1 may be occupied by anepitaxial silicon layer that extends to boundaries 3 and includes edgeportions 7 that overhang and extend over subjacent ledges formed inadjacent STI, shallow trench isolation, structures. Inner edges 5 areidentified by the dashed line which indicates the inwardly extendingedge of the ledge of the STI structure as will be shown more clearly inFIG. 1A. Transistor area 1 is defined, in the exemplary embodimentillustrated in FIG. 1, by opposed ends 9 and opposed ends 11. Opposedends 9 are parallel to one another and generally orthogonal to opposedends 11 but this is intended to be exemplary only and in various otherexemplary embodiments, transistor area 1 may take on various otherquadrilateral or other geometric shapes. Gate electrode 15 and spacers17 form elements of a transistor formed in transistor area 1 and havinga transistor channel with channel length direction 21.

Although transistor area 1 is illustrated to include edge portions 7along each of opposed sets of edges 9 and 11, such is intended to beexemplary only and in other exemplary embodiments, the edge portions 7that may overhang a ledge of an STI structure and extend past inneredges 5, may be present in less than all four of the opposed edges. Inone exemplary embodiment, edge portions 7 may be present only along thetransistor channel length direction 21 or they may be present only alongthe transistor channel width direction, orthogonal to channel lengthdirection 21.

Now referring to FIG. 1A as well as FIG. 1, transistor 25 includes gateelectrode 15, spacers 17, gate dielectric 29 and is formed oversubstrate 27. Substrate 27 is a semiconductor material and may besilicon according to one exemplary embodiment, although other suitablesemiconductor materials may be used as substrate 27 in other exemplaryembodiments. Transistor 25 includes source/drain regions 33 which areformed in substrate 27 and also in silicon layer 35. Silicon layer 35 isadvantageously an epitaxially formed silicon layer and is undoped atdeposition. Transistor channel 39 is disposed directly underneath gateelectrode 15. Silicon layer 35 includes edge portions 7 that extend overledges 41 of STI structures 43 and provide a T-shaped cross-section tothe transistor substructure. STI structures 43 each include a plateauwith top surface 45 which is substantially co-planar with the uppersurface of silicon layer 35 in the exemplary embodiment illustrated inFIGS. 1A and 1B. Boundaries 3 represent the intersection between topsurface 45 of STI structure 43 and silicon layer 35. Channel dopantimpurity region 73 is formed within substrate 27 including in transistorchannel 39. Concentrated dopant impurity regions 49 may be formed usingangled ion implantation techniques such as halo implantation, andadvantageously improve short channel effects for short gate lengths.Concentrated dopant impurity regions 49 may include a greaterconcentration of the dopant impurity present in channel dopant impurityregion 73. The gate length is the dimension underneath gate electrode 15along channel length direction 21. Gate length may range from about10-50 nanometers according to various exemplary embodiments and may beabout 30 nm in one exemplary embodiment.

FIG. 1B is a cross-sectional view taken along line 1B-1B of FIG. 1 andshows the width of the transistor channel. The effective width, Weff 13,extends from one boundary 3 to the opposed boundary 3 and is greaterthan a channel width extending from one inner edge 5 to the opposedinner edge 5, the width of original substrate surface 47 beneath siliconlayer 35. In one exemplary embodiment, Weff 13 may be 5-10% greater thanthe effective channel width that extends from inner edge 5 to opposedinner edge 5. Weff 13 may be about 0.2 microns and may range from about180-270 nanometers in various exemplary embodiments.

FIG. 2A shows STI structure 43 formed within semiconductor substrate 27.STI structure 43 may be formed using various known and future developedmeans. Nitride layers 53 and 55 are formed over substrate surface 47 andoxide layer 57 is interposed between nitride layers 53 and 55. Variousnitride materials and oxide materials may be used and variousthicknesses may be used. A nitride removal operation is used to removenitride layer 55 and produce the structure shown in FIG. 2B. Variousconventional wet or other etches may be used.

FIG. 2C shows the structure of FIG. 2B after an isotropic oxide removaloperation has been used to remove oxide layer 57 and portions 61 of STIstructure 43 (see FIG. 2B) to produce STI structure 43 with a terracedconfiguration. Various wet oxide etching operations may be used.

A nitride etching operation is performed upon the structure shown inFIG. 2C to remove nitride layer 53 and is followed by an oxide dip, i.e.etching operation to remove some oxide and produce the structure shownin FIG. 2D in which STI structure 43 has a central plateau that includestop surface 45 disposed above substrate surface 47 of substrate 27. STIstructure 43 includes a terraced structure with ledges 41 that extendoutwardly past plateau edges 69 of top surface 45. The rounded nature ofthe profile of terraced STI structure 43 between ledge 41 and thecentral plateau with top surface 45, is exemplary and ledge 41 may becharacterized by a sharper inner edge profile in other exemplaryembodiments such as in FIG. 3A.

FIG. 3A shows a portion of substrate 27 disposed between two opposedterraced STI structures 43. STI structures 43 include top surface 45with plateau edges 69 and inwardly facing ledges 41 that terminate atinner edges 5. A channel implantation operation may be carried out uponthe structure shown in FIG. 3A to produce the structure shown in FIG. 3Bwhich includes channel dopant impurity region 73. A channel implantationoperation utilizing a comparatively low energy, for example, an energythat may be about 10-30 KeV may be used according to one exemplaryembodiment. The channel ion implantation operation introduces dopantimpurities through substrate surface 47 and into substrate 27. Thechannel ion implantation operation may be used to introduce N-typedopant impurities according to one exemplary embodiment or P-type dopantimpurities according to another exemplary embodiment. According to oneexemplary embodiment, a BF₂ species may be implanted using animplantation energy of about 5-15 KeV to produce the channel dopantimpurity region 73 having boron as the dopant impurity. According toanother exemplary, PMOS, embodiment As may be the channel dopantimpurity species but other n-type or p-type channel dopant impurityspecies may be used in other exemplary embodiments. Channel dopantimpurity region 73 may include various concentrations of dopantimpurities therein.

An epitaxial silicon growth operation is then performed upon thestructure shown in FIG. 3B to produce the structure shown in FIG. 3C.The structure of FIG. 3C includes substrate 27 with channel dopantimpurity region 73 and epitaxial silicon layer 75. Epitaxial siliconlayer 75 may include a thickness 77 ranging from about 10-20 nanometersin various exemplary embodiments and grows laterally past inner edges 5and over ledges 41 as shown in the exemplary embodiment, to produce edgeportion 7 which may include a width of about 5-10 nanometers in variousexemplary embodiments. Various suitable conditions for the epitaxialgrowth of silicon may be used to form epitaxial silicon layer 75. It canbe seen that, as deposited, epitaxial silicon layer 75 does not includethe dopant impurities present in channel dopant impurity region 73.

A transistor is then formed upon the structure shown in FIG. 3C toproduce the structure shown in FIG. 3D. Gate electrode 15 and spacers 17may be formed over gate dielectric 29 and conventional ion implantationand/or diffusion operations may be used to form source/drain regions 33that are formed within both epitaxial silicon layer 75 and substrate 27.Conventional methods may be used to form source/drain regions 33. A haloor other angled ion implantation operations may be used to formconcentrated dopant impurity regions 49 which may alternatively bedescribed as a halo impurity region or a pocket impurity region. Thehalo ion implantation operation is a low energy, low currentimplantation carried out at a large incident angle so that implanteddopants penetrate underneath the edge of the gate electrode 15 tosuppress punch-through effects. The halo ion or other angled ionimplantation operation is used to introduce the same type dopants aswithin channel dopant impurity region 73 and opposite the dopantimpurity type used to form source/drain regions 33. The presence ofconcentrated dopant impurity regions 49 improves short channel effectsof transistor 25. Prior to annealing, the structure shown in FIG. 3Dincludes epitaxial silicon layer 75 being substantially deficient of thedopant impurities present in channel dopant impurity region 73 and inconcentrated dopant impurity regions 49. In particular, transistorchannel 39 of transistor 25 will include the channel dopant impurityspecies in channel dopant impurity region 73 within substrate 27 but thechannel dopant impurity species is essentially deficient from epitaxialsilicon layer 75 in transistor channel 39.

An annealing or other thermal activation operation may be then carriedout to drive some of the channel dopant impurities from channel dopantimpurity region 73 of substrate 27 into epitaxial silicon layer 75. Evenafter the annealing or other thermal activation operation is carriedout, the concentration of the channel dopant impurity species is reducedand comparatively less at the interface between transistor channel 39and gate dielectric 29 than the dopant concentration of the channeldopant impurity species as the depth into substrate 27 increases. Theconcentration of the dopant impurity species is greater in substrate 27than in epitaxial silicon layer 75, in transistor channel 39.

FIG. 4 is a graphical representation showing the profile of the dopantconcentration of the channel dopant impurity species within transistorchannel 39 as a function of depth into substrate 27. At depth=0,representing the top surface of transistor channel 39 and itsintersection with gate dielectric 29, the concentration is minimal andin the exemplary embodiment is shown to be less than 1e18. This isintended to be exemplary only and various other dopant concentrationsand profiles may be achieved in other exemplary embodiments. FIG. 4 ispresented to illustrate that dopant profile 83 according to the presentdisclosure differs from conventional dopant profile 81 in that theconcentration of the channel dopant impurity species is less at depth=0of transistor channel 39. Curve maxima 85 of dopant profile 83 residesin substrate 27 portion of transistor channel, further showing thatdopant impurity concentration is greater in substrate 27 than inepitaxial silicon layer 75.

According to one aspect, the disclosure provides a semiconductor devicecomprising a transistor area formed on a semiconductor substrate andcomprising a silicon layer disposed on the semiconductor substrate andhaving at least opposed edges that extend over a ledge of acorresponding STI, shallow trench isolation, structure and are boundedby a top surface of the corresponding STI structure; and a transistor.The transistor includes source/drain regions formed in the silicon layerand in the semiconductor substrate and a gate disposed over the siliconlayer including over the opposed edges.

According to one aspect, the disclosure provides a semiconductor devicecomprising a transistor area formed on a semiconductor substrate anddefined by a first set of opposed edges and a second set of opposededges orthogonal to the first set. Each edge is bounded by acorresponding STI, shallow trench isolation, structure. A silicon layeris disposed over the semiconductor substrate in the transistor area andincluding overhang portions disposed over respective inwardly facingledges of the corresponding STI structures and edges bounded byrespective top surfaces of the corresponding STI structures; and atransistor including source/drain regions is formed in the silicon layerand in the semiconductor substrate. A gate is disposed over the siliconlayer including over the opposed edges.

According to yet another aspect, a method for forming a semiconductortransistor is provided. The method comprises: providing a semiconductorsubstrate with a substrate surface; identifying a transistor area on thesemiconductor substrate having transistor area boundaries, thetransistor area boundaries including at least two opposed edges; andforming a shallow trench isolation, STI, structure along each of theopposed edges, each STI structure including a central plateau includinga top surface, and lower ledges that extend outwardly from the centralplateau and into the transistor area. The method further comprisesforming a silicon layer over the transistor area and including overhangportions disposed over the lower ledges and an upper surface bounded bythe central plateau along each of the opposed edges; and forming atransistor on the silicon layer including a gate that extends from oneopposed edge to the other of the opposed edges and over each of theoverhang portions.

The preceding merely illustrates the principles of the disclosure. Itwill thus be appreciated that those skilled in the art will be able todevise various arrangements which, although not explicitly described orshown herein, embody the principles of the disclosure and are includedwithin its spirit and scope. Furthermore, all examples and conditionallanguage recited herein are principally intended expressly to be onlyfor pedagogical purposes and to aid the reader in understanding theprinciples of the disclosure and the concepts contributed by theinventors to furthering the art, and are to be construed as beingwithout limitation to such specifically recited examples and conditions.Moreover, all statements herein reciting principles, aspects, andembodiments of the disclosure, as well as specific examples thereof, areintended to encompass both structural and functional equivalentsthereof. Additionally, it is intended that such equivalents include bothcurrently known equivalents and equivalents developed in the future,i.e., any elements developed that perform the same function, regardlessof structure.

This description of the exemplary embodiments is intended to be read inconnection with the figures of the accompanying drawing, which are to beconsidered part of the entire written description. In the description,relative terms such as “lower,” “upper,” “horizontal,” “vertical,”“above,” “below,” “up,” “down,” “top” and “bottom” as well asderivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,”etc.) should be construed to refer to the orientation as then describedor as shown in the drawing under discussion. These relative terms arefor convenience of description and do not require that the apparatus beconstructed or operated in a particular orientation. Terms concerningattachments, coupling and the like, such as “connected” and“interconnected,” refer to a relationship wherein structures are securedor attached to one another either directly or indirectly throughintervening structures, as well as both movable or rigid attachments orrelationships, unless expressly described otherwise.

Although the disclosure has been described in terms of exemplaryembodiments, it is not limited thereto. Rather, the appended claimsshould be construed broadly, to include other variants and embodimentsof the disclosure, which may be made by those skilled in the art withoutdeparting from the scope and range of equivalents of the disclosure.

What is claimed is:
 1. A semiconductor device comprising: a transistorarea formed on a semiconductor substrate and comprising a silicon layerdisposed on a silicon region of said semiconductor substrate and havingat least opposed edges that each extend over a ledge of a correspondingSTI, shallow trench isolation, structure and are bounded by a topsurface of said corresponding STI structure; and a transistor includingsource/drain regions formed in said silicon layer and in said siliconregion, and a gate disposed over said silicon layer including over saidopposed edges and further comprising a channel dopant impurity presentbeneath said gate in a first concentration in said silicon region ofsaid semiconductor substrate and in a second concentration less thansaid first concentration in said silicon layer, wherein each of saidsilicon layer and said silicon region are comprised entirely of silicon.2. The semiconductor device as in claim 1, wherein said transistor areais defined by said opposed edges and a set of opposed further edgesdisposed orthogonal to said opposed edges, each of said further edgesextending over a further ledge of a corresponding further STI structureand bounded by a top surface of said corresponding further STIstructure.
 3. The semiconductor device as in claim 2, wherein saidtransistor includes a channel width direction extending from one of saidopposed edges to the other of said opposed edges and a channel lengthdirection orthogonal to said channel width direction.
 4. Thesemiconductor device as in claim 1, wherein said STI structures areterraced structures such that said STI structures include a centralplateau having said top surface coplanar with a top surface of saidsilicon layer.
 5. The semiconductor device as in claim 1, wherein saidsemiconductor substrate comprises a silicon substrate, said siliconregion is an upper portion of said semiconductor substrate, and saidtransistor includes a gate dielectric and a channel formed in saidsemiconductor substrate and in said silicon layer.
 6. The semiconductordevice as in claim 1, wherein said transistor includes a channel in saidsilicon layer.
 7. The semiconductor device as in claim 1, wherein saidtransistor includes a channel width direction extending from one of saidopposed edges to the other of said opposed edges and said channel widthincluding said edges is about 10 percent greater than a distance betweeninner boundaries of said ledges of said STI structures along saidchannel width direction.
 8. The semiconductor device as in claim 1,further comprising a duality of concentrated regions of said channeldopant impurities present at a second concentration at lateral edges ofsaid gate, said second concentration greater than said firstconcentration.
 9. A semiconductor device comprising: a transistor areaon a silicon substrate and defined by a first set of opposed edges and asecond set of opposed edges orthogonal to said first set of opposededges, each said edge bounded by a corresponding STI, shallow trenchisolation, structure; a silicon layer disposed on said silicon substratein said transistor area and including overhang portions disposed overrespective inwardly facing ledges of said corresponding STI structuresand borders bounded by respective top surfaces of said corresponding STIstructures; and a transistor including source/drain regions formed insaid silicon layer and in said silicon substrate, a gate disposed oversaid silicon layer including over said opposed edges, and a channelregion disposed beneath said gate, said channel region including arelatively higher concentration of channel dopant impurities in saidsilicon substrate and a relatively lower concentration of said channeldopant impurities in said silicon layer, wherein each of said siliconlayer and said silicon substrate are comprised entirely of silicon. 10.The semiconductor device as in claim 9, wherein an effective channelwidth of said transistor includes a width of said silicon layerincluding said overhang portions and is about 10% greater than adistance between said inwardly facing ledges along said gate.
 11. Thesemiconductor device as in claim 9, wherein said channel dopantimpurities comprise boron.